U.S. marketers spent 14 percent more in online advertising in the first half of the year, compared with the same period in 2011, hitting $17 billion, according to a study sponsored by the Interactive Advertising Bureau (IAB) and conducted by PwC US.

U.S. marketers spent 14 percent more in online advertising in the first half of the year, compared with the same period in 2011, hitting $17 billion, according to a study sponsored by the Interactive Advertising Bureau (IAB) and conducted by PwC US.

It's the highest amount ever spent on online ads during a year's first semester since the report was first issued in 1996, although the growth failed to match the 23 percent registered during 2011's first half. At the height of the economic crisis, this market shrunk 5 percent in the first half of 2009.

In what will be good news for Google in particular, search remained the biggest ad category, accounting for 48 percent of all spending, up 2 percentage points from last year's first half. Search ad spending grew 19 percent.

In a statement issued Thursday, IAB and PwC officials said the results indicate that the U.S. online ad market is healthy and strong, with attractive growth opportunities in mobile and video.

"This report establishes that marketers increasingly embrace mobile and digital video, as well as the entire panoply of interactive platforms to reach consumers in innovative and creative ways," said the IAB's President and CEO Randall Rothenberg in the statement.

The 10 biggest online ad sellers captured 73 percent of the revenue, so the market continues to be largely dominated by these vendors, which aren't mentioned in the report but include the usual suspects, like Google, Yahoo, Facebook, Microsoft and AOL.

Retailers as a group spent the most (20 percent), followed by financial services and automotive companies (each one with 13 percent) and telecom companies (12 percent.)